1. Field of the Invention
The present invention generally relates to a method for monitoring a curvature of a micro-lens in-line and, more particularly, to a method for monitoring a curvature of a micro-lens in-line during a production flow, in which a size of a circle image on a top view of the micro-lens is measured according to a change in a focus on an optic scope.
2. Discussion of the Background
Generally, a complementary metal oxide semiconductor (CMOS) image sensor can include a photo-sensing unit and a logic circuit unit. The photo-sensing unit may receive the light and the logic circuit unit may convert the light to electric signals. In order to improve photosensitivity, an occupying area of the photo-sensing unit in an entire CMOS image sensor may be increased. However, since the entire area of the CMOS image sensor is fixed, this method for increasing the occupying area of the photo-sensing unit has associated limits. Accordingly, light may be condensed to the photo-sensing unit by changing the path of light incident extending beyond the photo-sensing unit. For this, a micro-lens can be provided that corresponds to the photo-sensing unit on a color filter array.
Referring now to FIG. 1, a cross sectional view illustrating a conventional CMOS image sensor is shown. In a process for fabricating a micro-lens of a CMOS image sensor according to the conventional approach of FIG. 1, a photo-sensing unit (not shown, for example, a photodiode) and a logic circuit unit (not shown, for example, a plurality of transistors) may be formed in each cell region on substrate 101.
Then, a color filter array layer 102 may be formed in each cell region of the substrate 101 having the photo-sensing unit and the logic circuit unit. In color filter array layer 102, each color filter layer of red, green and blue may be provided corresponding to each photo-sensing unit, for example. Next, planarization layer 103 may be formed on color filter array layer 102. Further, micro-lens 104 may be formed on planarization layer 103 corresponding to each photo-sensing unit.
According to the above process steps, a conventional CMOS image sensor may be fabricated. In these conventional fabrication process steps, the last process steps for fabricating color filter array layer 102 and micro-lens 104 may be directly related to the color characteristics of the CMOS image sensor. In particular, a curvature of each micro-lens may be a very important factor in the CMOS image sensor operation. However, in many fabrication process lines, it is difficult to monitor the curvature of the micro-lens. Accordingly, many such fabrication lines are typically controlled by measuring a space and not a curvature of a sample micro-lens.
Referring now to FIG. 2, an illustration of a photograph of measuring the space between micro-lenses by a conventional CD-SEM approach is shown. As described above, on such a fabrication process line, it may be very difficult or not feasible to measure the curvature of the micro-lens. In the conventional approach indicated in FIG. 2, in the fabrication process line, the space between micro-lenses may be measured by CD-SEM, thereby allowing control of the fabrication process.
Further, since it may be difficult or impractical to measure the curvature of a micro-lens on a fabrication process line, the curvature of the micro-lens may instead be determined by using a cross-sectional SEM image. In such an approach, the substrate of a completed CMOS image sensor is generally destroyed in order to make the appropriate measurements after completion of the micro-lens fabrication process.
Referring now to FIG. 3, an illustration of a conventional process for monitoring a curvature of a micro-lens by rejecting a produced substrate is shown. In the approach indicated in FIG. 3, in order to measure the curvature of a sample micro-lens, a produced substrate are typically rejected or destroyed in order to provide samples at fixed periods in the manufacturing process flow. Each such sample can then be used to obtain a cross-sectional SEM for measuring purposes. From the cross-sectional SEM, a horizontal length and a vertical height of the sample micro-lens may be measured to calculate the curvature of the micro-lens.
However, as discussed above, methods for fabricating a CMOS image sensor according to conventional approaches has several disadvantages. First, it may be difficult or impractical to monitor the curvature of a micro-lens, which is very important factor in CMOS image sensors. Accordingly, it is difficult to determine whether a process for manufacturing the micro-lens has any problems or is producing micro-lenses outside of design specifications.
Also, in order to monitor a curvature of a micro-lens in other conventional approaches, produced substrates may be rejected or destroyed at fixed periods in a fabricating process in order to obtain representative cross-sectional SEMs. In this approach, rejected substrates cannot be re-used, so there is typically a corresponding yield and/or productivity loss as a result.